This invention relates to switching and, more particularly, to optical switching associated with communication and/or computing.
The interest attracted by optical digital switching and computing is mainly stimulated by its potential to implement massively parallel architectures. This holds especially true for free space systems where logic arrays of 100 by 100 logic devices or more can be connected through imaging setups. Free-space propagations also offers the potential to do the communications within such a computer at an extremely high temporal bandwidth without introducing problems such as clock skew or crosstalk.
The use of optical imaging setups for parallel interconnects, however, restricts the variety of feasible topologies to networks that possess an interconnection pattern that is regular, because one would wish to treat an entire image array (comprising many light rays) as a single beam. On the other hand, even when networks with regular connection patterns are more difficult to use in computing applications the high space-bandwidth product of an optical system, i.e., the exceptionally large number of connections that can be established concurrently, offers the potential for reducing the overall complexity of the interconnection scheme. For this reason, interest has grown in regular interconnection networks such as the perfect shuffle or the banyan. These networks are sometimes referred to as alignment networks. Alignment networks have been used in digital processing for implementing fast algorithms, and a number of publications exist about the use of such networks in optical systems. See, for example, A. Huang, "Architectural Considerations Involved in the Design of an Optical Digital Computer," Proc. IEEE 72, No. 7, (1984) 780-786; and H. S. Stone "Parallel Processing with the Perfect Shuffle" IEEE Transactions Comp. C-20, No. 2 (1971) 153-161.
The regularity of alignment networks such as the perfect shuffle or the banyan does, in fact, seem to limit the flexibility of designing a digital general purpose computer. A circuit with a specific interconnection pattern is sometimes difficult to wrest from a network with regular connectivity. It has been shown, however, that these networks can be used for general purpose computers efficiently in terms of gate count and throughput. See, for example, Murdocca et al. "Optical Design of Programmable Logic Arrays," Applied Optics May 1988.
On a computational level, the perfect shuffle and the banyan are isomorphic. From a systems point of view, however, the implementations of different (though isomorphic) alignment networks imposes different problems. These arise mainly from the fact that, in general, alignment networks are spacevariant, which means that the interconnection pattern is dependent on the input position of the network node, or the pixel. That condition does not fit well with optics and, therefore, attempts to implement these networks with optic setups result in losses of light intensity and resolution.
A space variant network that exhibits a regular connectivity and can be implemented with essentially no light loss is the crossover network. One such network, designed for VLSI applications, has been described by Wise, in "Compact Layouts of Banyan/FFT Networks" VSLI Systems and Computations, Computer Science Press (1981) pp. 186-195. His proposal was motivated by the need to have wires of equal length in order to reduce path length differences between signals. FIG. 1 depicts a diagramatic representation of Wise's crossover network. The functional elements (10) are not important in describing the network. Their function can vary with the specific applications for which the network is used. Reflection elements (20) redirect the signal flow, to effect some of the crossover connections.
For an optical implementation of the FIG. 1 network, it is interesting to note that the signals emerge from all ports under the same angle and that shifts of different value are achieved by using a different separation between the rows of elements 10, with a corresponding change in the length of reflecting elements 20. Wise's network may be useful for a waveguide-optical implementation where the lines of the diagram in FIG. 1 represent the waveguides. For a free-space optical implementation, however, it is not very well suited, since optical waves traveling in free space cannot easily be confined in both direction and space.